Method and system for detecting dislodgement of an implanted right atrial endocardial lead using a sensing period derived from a ventrical lead

ABSTRACT

A system and method are provided for detecting dislodgment of a right atrial endocardial lead implanted within a heart right atrium wherein the lead includes an electrode. Heart activity of the right ventricle of the heart and heart activity with the endocardial lead electrode are sensed during a time period. The sensed heart activity is stored in memory. The stored heart activity is analyzed to establish a first time window from the sensed ventricle activity and a second time window spaced from and preceding the first time window. The heart activity sensed with the endocardial lead electrode during the first time window is then compared to the heart activity sensed with the endocardial lead electrode during the second time window. In another embodiment, whenever heart activity is sensed with the atrial endocardial lead during the first time period, the atrial endocardial lead is considered dislodged.

This application is a divisional of application Ser. No. 09/103,218,filed Jun. 23, 1998 now U.S. Pat. No. 6,067,469.

BACKGROUND OF THE INVENTION

The present invention generally relates to a method and system fordetecting dislodgment of an implanted right atrial endocardial lead. Thepresent invention is more particularly directed to such a system andmethod incorporated within an implantable atrial defibrillator having,in addition to atrial fibrillation cardioversion capability, at leastatrial pacing, and more particularly, atrial antitachycardia pacingcapability.

Atrial fibrillation is probably the most common cardiac arrhythmia.Although it is not usually a life-threatening arrhythmia, it isassociated with strokes thought to be caused by blood clots forming inareas of stagnant blood flow as a result of prolonged atrialfibrillation. In addition, patients afflicted with atrial fibrillationgenerally experience rapid and irregular beating of the heart and mayeven experience dizziness as a result of reduced cardiac output.

Atrial fibrillation occurs suddenly, and many times can only becorrected by discharging electrical energy into the atria of the heartof the patient. This treatment is preferably synchronized to a detectedR wave of the heart in order to avoid shocking the atria during the Twave or vulnerable period of the heart. The amount of energy which maybe required to successfully cardiovert the atria can be as low as onejoule and as high as six joules. In most cases, energy of about two tofour joules is required to cardiovert atrial fibrillation back to normalsinus rhythm (NSR).

Atrial tachycardia is a less common accelerated atrial arrhythmia. It isa more organized arrhythmia than atrial fibrillation characterized by avery fast and substantially constant atrial rate. Atrial tachycardiaresults in many of the same symptoms as atrial fibrillation. It canresult in dizziness, shortness of breath and a rapid ventricular rate.

Implantable atrial defibrillators are known which detect the presence ofatrial fibrillation and provide a single cardioverting pulse ofelectrical energy to the atria when atrial fibrillation is detected.Usually, the therapy is applied in synchrony with a detected R wave toavoid therapy application during the ventricular vulnerable period ofthe heart thereby preventing the induction of a lethal ventriculararrhythmia.

Antitachycardia atrial pacing is often used as a therapy for atrialtachycardia. In such therapy, the atria are paced at a rate faster thanthe atrial intrinsic rate for a time to terminate the atrialtachycardia. This is commonly known as overdrive pacing.

To accomplish overdrive pacing, the right atrium is commonly paced at ahigh rate by an implanted device. The implanted device applies pacingpulses to the right atrium with a pair of small, closely spaced,electrodes, generally referred to as a bipolar electrode pair. Theelectrodes are carried on an implantable endocardial lead. The lead isdesigned for fixation in the right atrium, either by tines or by ahelical screw-in tip which can serve as one of the electrodes. Bothforms of fixation are well known in the art.

One potential problem with atrial antitachycardia pacing is aninadvertent dislodgment of the right atrial endocardial lead. Should thelead become dislodged, the atrial pacing electrodes could be displacedto a position close to or within the right ventricle. High rateantitachycardia pacing intended for the right atrium with the lead insuch a dislodged position could cause the ventricles to be overdrivepaced. This presents the possibility of accelerating the ventricles intoa dangerous and potentially life threatening arrhythmia.

Atrial antitachycardia pacing can still be a very useful therapy. Asignificant number of atrial fibrillation patients suffer from thisarrhythmia. Hence, it would be advantageous to be able to incorporatethis therapy into an implantable atrial defibrillator. By virtue of thepresent invention, atrial antitachycardia pacing may be incorporatedwithin implantable devices, such as an atrial defibrillator.

SUMMARY OF THE INVENTION

The invention provides a method of detecting dislodgment of a rightatrial endocardial lead implanted in the right atrium of a heart, thelead having at least one electrode. The method includes the steps ofsensing ventricular activity of the heart with an electrode pair withinthe right ventricle of the heart establishing a first sensing periodresponsive to the ventricular activity sensed with the electrode pair,establishing a second sensing period spaced from and preceding the firstsensing period established responsive to the ventricular activity sensedwith the electrode pair sensing activity of the heart with the at leastone electrode of an implanted endocardial lead during the first sensingperiod and the second sensing period, and comparing the heart activitysensed with the at least one electrode during the first sensing periodto the heart activity sensed with the at least one electrode during thesecond sensing period.

The invention further provides a method of detecting dislodgment of aright atrial endocardial lead implanted in the right atrium of a heart,the lead having at least one electrode. The method includes the steps ofsensing ventricular activity of the heart with an electrode pair withinthe right ventricle of the heart, establishing a sensing periodresponsive to the ventricular activity sensed with the electrode pair,and sensing for activity of the heart with the at least one electrode ofthe implanted endocardial lead during the sensing period. The inventionfurther provides a system for detecting dislodgment of an implantedright atrial endocardial lead having at least one electrode. The systemincludes means including an electrode pair for sensing ventricularactivity of the heart within the right ventricle of the heart, means forestablishing a first sensing period responsive to the ventricularactivity sensed with the electrode pair, and means for establishing asecond sensing period spaced from and preceding the first sensing periodestablished responsive to the ventricular activity sensed with theelectrode pair, the system further includes means for sensing activityof the heart with the at least one electrode of the implantedendocardial lead during the first sensing period and the second sensingperiod, and means for comparing the heart activity sensed with the atleast one electrode during the first sensing period to the heartactivity sensed with the at least one electrode during the secondsensing period. The invention still further provides a system fordetecting dislodgment of an implanted right atrial endocardial leadhaving at least one electrode. The system includes means including anelectrode pair for sensing ventricular activity of the heart within theright ventricle of the heart, means for establishing a sensing periodresponsive to the ventricular activity sensed with the electrode pairand means for sensing activity of the heart with the at least oneelectrode of the implanted endocardial lead during the sensing period.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The invention,together with further objects and advantages thereof, may best beunderstood by making reference to the following description taken inconjunction with the accompanying drawing, in the several figures ofwhich like reference numerals identify identical elements, and wherein;

FIG. 1 is a schematic block diagram of a fully implantable atrialdefibrillator embodying the present invention including a pacer andassociated right atrial endocardial lead for applying atrialantitachycardia pacing to a heart; and

FIG. 2 is a series of electrograms illustrating the present invention inaccordance with a preferred embodiment thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Prior to referring to FIG. 1, a general description of a typical ornormal cardiac cycle may be helpful in understanding the operation andvarious aspects of the present invention. The beginning of a cardiaccycle in normal sinus rhythm is initiated by a P wave which is normallya small positive wave. The P wave induces depolarization of the atria ofthe heart. Following the P wave there is a cardiac cycle portion whichis substantially constant having a time duration on the order of, forexample, 120 milliseconds.

The QRS complex of the cardiac cycle then normally occurs after thesubstantially constant portion. The dominating feature of the QRScomplex is the R wave which is a rapid positive or negative deflection.The R wave generally has an amplitude greater than any other wave of thecardiac cycle and is characterized by a rapid deviation from and returntoward baseline. The R wave is the depolarization of the ventricles. TheQRS complex is completed by the S wave which is generally a smalldeflection which returns the cardiac cycle to baseline.

Following the S wave of the QRS complex, the T wave occurs which isseparated from the QRS complex by about 250 milliseconds. The T wave isrelatively long in duration of, for example, on the order of 150milliseconds. The cardiac cycle segment between the S wave and the Twave is commonly referred to as the ST segment.

The next cardiac cycle begins with the next P wave. The duration of acardiac cycle may be on the order of 800 milliseconds.

Referring now to FIG. 1, it illustrates a fully implantable atrialdefibrillator 30 embodying the present invention shown in associationwith a schematically illustrated human heart 10. The portions of theheart 10 illustrated in the sole figure are the right ventricle 12, theleft ventricle 14, the right atrium 16, the left atrium 18, the superiorvena cava 20, the coronary sinus channel 21 which, as used herein,denotes the coronary sinus 22 and the great cardiac vein 23, thecoronary sinus ostium or opening 24, and the inferior vena cava 27.

The atrial defibrillator 30 generally includes an enclosure 32 forhermetically sealing the internal circuit elements of the atrialdefibrillator to be described hereinafter, a ventricular endocardial orfirst lead 34, a right atrial endocardial or second lead 35 and anintravascular or third lead 36. The enclosure 32 and leads 34, 35, and36 are arranged to be implanted beneath the skin of a patient so as torender the atrial defibrillator 30 fully implantable.

The intravascular lead 36 generally includes a first or tip electrode 44and a second proximal electrode 46. As illustrated, the lead 36 isflexible and arranged to be passed down the superior vena cava 20, intothe right atrium, into the coronary sinus ostium 24, and advanced intothe coronary sinus channel 21 of the heart near the left side thereof sothat the first or tip electrode 44 is within the coronary sinus channel21 either within the coronary sinus 22 adjacent the left ventricle 14and beneath the left atrium 18 or most preferably within the greatcardiac vein 23 adjacent the left ventricle 14 and beneath the leftatrium 18. The electrodes 44 and 46 are spaced apart such that when thefirst electrode 44 is positioned as described above, the secondelectrode 46 is in the right atrium 16. The first electrode 44 togetherwith the second electrode 46 provide bi-polar sensing of heart activityin the atria 16 and 18. The first electrode 44 and the second electrode46 further provide for the delivery of defibrillating electrical energyto the atria. The electrodes 44 and 46 are preferably elongatedcardioverting electrodes.

The first lead 34 preferably comprises a ventricular endocardial leadhaving bi-polar pair electrodes 38 and 40 arranged for establishingelectrical contact with the right ventricle 12 of the heart 10. Theelectrodes 38 and 40 permit bi-polar sensing of ventricular activationsin the right ventricle and pacing in the right ventricle. Asillustrated, the lead 34 is fed through the superior vena cava 20, intothe right atrium 16, and then into the right ventricle 12.

The second lead 35 preferably comprises a right atrial endocardial leadhaving bi-polar pair electrodes 37 and 39. Electrode 39 preferably is ahelical screw-in coil for both providing fixation of the lead 35, asknown in the art, and establishing electrical contact with the rightatrium 16 of the heart 10. The electrodes 37 and 39 permit localizedbi-polar sensing of heart activity in the right atrium and pacing,including overdrive antitachycardia pacing, in the right atrium. Asillustrated, the lead 35 is fed through the superior vena cava 20 andinto the right atrium 16.

Within the enclosure 32, the atrial defibrillator 30 includes a firstsense amplifier 50, a second sense amplifier 51, and a third senseamplifier 54 and an R wave detector 52. The first sense amplifier 50forms an RV channel which provides an electrogram of the sensed rightventricular heart activity at an input of multiplexer 45 and to an inputof an R wave detector 52. The second sense amplifier 51 forms an RAchannel to provide an electrogram of the sensed right atrial heartactivity at its output which is coupled to another input of multiplexer45. The third sense amplifier 54 forms an RACS channel to provide anelectrogram of the sensed right atrium to left atrium heart activity atits output which is coupled to another input of the multiplexer 45 andan atrial activity detector 60. The sense amplifiers may include adifferentiating filter so that the electrograms which they provide aredifferentiated electrogram signals.

The R wave detector 52 provides one or more output pulses for each Rwave sensed during a cardiac cycle of the heart. To that end, the R wavedetector may include a further differentiating filter fordifferentiating the differentiated cardiac signal provided by senseamplifier 50 resulting in a twice differentiated second cardiac signal.The R wave detector 52 may further include a threshold circuit forsetting an upper and lower threshold which provides an output when thetwice differentiated second cardiac signal transitions beyond either theupper or lower thresholds.

Finally, the R wave detector preferably further includes an output pulserate limiter (not shown) having a programmable pulse repetition timeinterval. The pulse repetition time interval is set to be as short aspossible to allow detection of the last threshold crossing for an Rwave. The R wave detector 52 thus provides at least one such pulse toindicate the beginning of each detected R wave and one such pulse toindicate the completion of each detected R wave so that the beginningand end of each R wave may be determined.

The atrial activity detector 60 also preferably includes a similarfurther differentiating filter (not shown) and output pulse rate limiter(not shown). This similarly enables the atrial activity detector 60 toprovide at least one output pulse to indicate the beginning of eachsensed P-wave and another to indicate the end of each sensed P wave.

The enclosure 32 of the atrial defibrillator 30 further includes amicroprocessor 62. The microprocessor 62 is preferably implemented inaccordance with this embodiment of the present invention to result in aplurality of functional stages. The stages include a comparator stage63, an R window stage 64, a timer 65, a P window stage 66, a P peakstage 67 an R peak stage 68, an atrial arrhythmia detector in the formof an atrial fibrillation detector 70 and a charge delivery and energycontrol stage 74.

The microprocessor 62 is arranged to operate in conjunction with amemory 90 which is coupled to the microprocessor 62 by a multiple-bitaddress bus 94 and a bi-directional multiple-bit data bus 96. Thispermits the microprocessor 62 to address desired memory locations withinthe memory for executing write or read operations. During a writeoperation, the microprocessor stores data, such as time stamps, oroperating parameters, in the memory at the addresses defined bymultiple-bit addresses conveyed over the address bus 94 and conveys theoperating parameters and data to the memory 90 over the multiple-bitdata bus 96. During a read operation, the microprocessor 62 obtains dataor operating parameters from the memory at the storage locationsidentified by the multiple-bit addresses provided over the address bus94 and receives the operating parameters and data from the memory overthe bi-directional data bus 96.

For entering operating parameters into the memory 90, the microprocessor62 receives the programmable operating parameters from an externalcontroller 100 which is external to the skin of the patient. Theexternal controller 100 is arranged to communicate with areceiver/transmitter 102 within enclosure 32 which is coupled to themicroprocessor 62 over a bi-directional bus 104. Thereceiver/transmitter 102 conveys various information which it obtainsfrom the microprocessor 62 to the external controller 100 or receivesprogramming parameters from the external controller 100 which thereceiver/transmitter 102 then conveys to the microprocessor 62 forstorage in memory 90.

The receiver/transmitter 102 includes a transmitting coil 106 so thatthe receiver/transmitter 102 and coil 106 form a communication means.Such communication means are well known in the art and may be utilizedas noted above for receiving commands from the external controller 100and for transmitting data to the external controller 100. One preferredcommunication system is disclosed in U.S. Pat. No. 5,342,408 whichissued on Aug. 30, 1994 for “Telemetry System for an Implantable CardiacDevice,” which patent is assigned to the assignee of the presentinvention and incorporated herein by reference.

The atrial defibrillator 30 further includes an analog to digitalconverter 57 and a direct memory access controller (DMA) 59. The analogto digital converter 57 has an input coupled to the output of themultiplexer 45 for receiving the electrogram signals generated by thesense amplifiers 50, 51, and 54. During a data acquisition, the analogto digital converter 57 converts the electrogram signals into digitaldata. The digital data is received by the DMA 59 which conveys thedigital data to memory 90 over a data bus 59 a for storage in memory atpredetermined locations selected by the DMA 59 over an address bus 59 b.The electrogram signals thus stored in digital form representingactivity of the heart are thereafter utilized by the microprocessor toperform various functions. For example, for atrial fibrillationdetection, the atrial fibrillation detector 70 utilizes the stored datafrom the RACS channel for detecting the presence of atrial fibrillationof the heart. For detecting dislodgment of the lead 35 in accordancewith a preferred embodiment of the present invention, it may utilizedigital electrogram data from each of the RV, RA, and RACS channels.

The atrial defibrillator 30 further includes a charger and storagecapacitor circuit 76 of the type well known in the art which charges astorage capacitor to a selected peak voltage and a discharge circuit 78for discharging the storage capacitor within circuit 76 for apredetermined time to provide a controlled discharge output ofelectrical energy when required to the atria of the heart. To that end,the discharge circuit 78 is coupled to the first electrode 44 and thesecond electrode 46 of lead 36 for applying the cardioverting ordefibrillating electrical energy to the atria. The defibrillator 30further includes a depletable power source 80, such as a lithiumbattery, for providing power to the electrical components of the atrialdefibrillator 30, and a real time clock 82.

The atrial defibrillator 30 lastly includes a pacer 55 which is coupledto electrodes 38 and 40 of lead 34 and to electrodes 37 and 39 of lead35. The pacer 55 preferably includes circuitry for sensing ventricularactivity with electrodes 38 and 40 and pacing circuitry for applyingpacing pulses to the ventricles with electrodes 38 and 40. Similarly,the pacer 55 preferably includes circuitry for sensing atrial activitywith electrodes 37 and 39 and pacing circuitry for applying pacingpulses including antitachycardia atrial overdrive pacing to the atriawith electrodes 37 and 39. Further, the pacer 55 may provide singlechamber pacing in either the right ventricle 12 or right atrium 16,asynchronously or on demand, or dual chamber pacing. Such pacers andmodalities are well known in the art. The pacer is coupled to themicroprocessor over a line 56 to permit the microprocessor to configurethe pacer 55 for any one of its pacing modalities including theantitachycardia atrial overdrive pacing modality.

To briefly describe the operation of the atrial defibrillator forcardioverting atrial fibrillation of the heart, an atrial fibrillationdetection is initiated by the sense amplifiers 50 and 54, the R wavedetector 52, the atrial activity detector 60, the analog to digitalconverter 57, the multiplexer 45 and the DMA 59 being enabled. A dataacquisition is first performed for a data acquisition period of, forexample, eight seconds. During the eight second data acquisition period,the electrogram signals from sense amplifiers 50 and 54 are digitized bythe analog to digital converter 57 into digital data and the digitaldata is caused to be stored in the memory 90 by the DMA 59 as previouslydescribed. Also during this time, each output of the R wave detector 52causes an interrupt to the microprocessor 62. Each R wave interrupt istime stamped and the interrupt time stamps are stored in the memory 90along with the digital data from DMA 59.

After the eight second data acquisition period is completed, the atrialfibrillation detector 70 is enabled and analyzes the stored electrogramdata from the RACS channel. The atrial fibrillation detector 70 maydetermine if the atria 16 and 18 are in fibrillation in a manner knownin the art as, for example, described in U.S. Pat. No. 5,486,199 whichissued on Jan. 13, 1996 for “System and Method For Reducing FalsePositives In Atrial Fibrillation Detection,” which patent is assigned tothe assignee of the present invention and incorporated herein byreference. If the atria are in fibrillation and thus in need ofcardioversion, the charge delivery control 74 causes the charger andstorage capacitor circuit 76 to charge the storage capacitor within thecircuit 76 to a selected peak voltage. After the capacitor is charged,another data acquisition is performed and the atrial fibrillationdetector 70 confirms the presence of atrial fibrillation. Thereafter,and in timed relation to a detected R wave, the discharge circuit 78,applies a portion of the stored electrical energy to electrodes 44 and46 and thus the atria 16 and 18 to cardiovert the atria 16 and 18.

In accordance with a preferred embodiment of the present invention,prior to detecting if the lead 35 has become dislodged, it is preferredthat a detection for atrial fibrillation be performed to make sure thatthe heart is in normal sinus rhythm. This may be performed as previouslydescribed by initiating a data acquisition followed by atrialfibrillation detection by the atrial fibrillation detector 70.

If the heart is in normal sinus rhythm, the lead dislodgment detectionis begun by first acquiring electrograms from the RA and RV sensechannels. This is accomplished by enabling sense amplifiers 50 and 51the multiplexer 45, the analog to digital converter 57, and the DMA 59.The electrogram data representing the RA and RV electrograms are storedin memory 90 by the DMA 59 during the electrogram acquisition. Theelectrogram acquisition need only last for one complete cardiac cycle.However, additional cycles may be desirable to afford checks onelectrogram signal quality, for example.

Next, from the RV electrogram data, the R window stage 64 analyzes thedata and identifies an R wave. This may be seen in FIG. 2 wherein theelectrogram 206 is the RV electrogram and the R wave 208 is identified.The R wave may be readily identified because the bi-polar electrode pair38 and 40 only senses localized activity. Very little far field activityis sensed by the RV channel.

Either from the analysis of the electrogram 206 or from time stampedinterrupt generated by the R wave detector 52, the first detected R waveevent is denoted as R start 209 by the R window stage 66. The R windowstage 66 then defines a first sensing period or R window 207 whichbegins on R start and ends a fixed time thereafter, for example, 100milliseconds after R start to assure completion of the R wave 208.

Next, the P window stage 66 identifies a P window 205 from the R window207. The P window is selected to terminate before the R window 207commences by, for example, 40 milliseconds. The beginning of the Pwindow is determined to begin about 150 to 200 milliseconds before itterminates. This establishes a P window wherein only P waves should bedetected by lead 46 if it is not dislodged.

Once the P window 205 and R window 207 are defined, the RA electrogram,obtained from electrodes 37 and 39 of lead 35, is analyzed during the Pwindow 205 and R window 207. More specifically, the RA electrogramportion during the P window 205 is compared to the RA electrogramportion during the R window 207. In accordance with this preferredembodiment, the absolute peak values of the RA electrogram during thesetime periods are compared to determine if lead 35 has been dislodgedwith electrodes 37 and 39 migrating near to or within the rightventricle 12. The absolute peak value of the RA electrogram during the Pwindow 205 (P peak) is determined by the P peak stage 67. The absolutepeak value of the RA electrogram during the R window 207 (R peak) isdetermined by the R peak stage 68. The peak values are then compared bythe comparator stage 63. If the comparison satisfies a predeterminedcriteria, the lead is considered to be dislodged and overdrive atrialantitachycardia pacing is disabled. For example, if R peak is largerthan a certain percentage, say 75%, of P peak, then lead 35 may beconsidered to be dislodged.

The above criteria accurately detects the dislodgment of lead 35. Sinceelectrodes 37 and 39 are a bi-polar electrode pair nominally within theright atrium, the RA electrogram generated by these electrodes shouldhave a relatively small peak amplitude during the R window as comparedto its peak amplitude during the P window. The Electrogram 210 of FIG. 2illustrates an exemplary RA electrogram for a nondislodged lead 35. Hereit can be seen that the R peak 212 is much less than 75% of P peak 214.However, electrogram 216 of FIG. 2 illustrates an exemplary RAelectrogram for a dislodged lead 35. Here, R peak 218 is much largerthan 75% of P peak 220. Of course, other features of the RA electrogramduring the P window and R window or peak comparative factors may be usedfor comparison in accordance with the broader aspects of the presentinvention as would be appreciated by those skilled in the art.

As an alternative embodiment in accordance with the broader aspects ofthe present invention, the establishment of the second sensing period205 may be dispensed with. In accordance with this embodiment, only theR window 207 need be established from the ventricular activity sensedwith the electrode pair 38, 40. Then, if any cardiac activity is sensedin the RA channel with the electrode pair 37, 39 during this sensingperiod, the lead 46 may be considered dislodged. In this event,overdrive pacing of the atria should be disabled.

The detection for lead dislodgment may be performed at spaced aparttimes such as periodically. This may be accomplished by the timer 65activating the detection sequence once each day, for example.Alternatively, the timer 65 may be used to periodically initiate anatrial fibrillation detection for rhythm classification followed by alead dislodgment detection in accordance with the present invention.

While particular embodiments of the present invention have been shownand described, modifications may be made, and it is therefore intendedin the appended claims to cover all such changes and modifications whichfall within the true spirit and scope of the invention.

What is claimed is:
 1. A method of detecting dislodgment of a rightatrial endocardial lead implanted in the right atrium of a heart, thelead having at least one electrode, said method including the steps of:sensing ventricular activity of the heart with an electrode pair withinthe right ventricle of the heart; establishing a first sensing periodresponsive to the ventricular activity sensed with the electrode pair;establishing a second sensing period spaced from and preceding the firstsensing period established responsive to the ventricular activity sensedwith the electrode pair; sensing activity of the heart with the at leastone electrode of the implanted endocardial lead during the first sensingperiod and the second sensing period; and detecting dislodgment of theright atrial endocardial lead by comparing the heart activity sensedwith the at least one electrode during the first sensing period to theheart activity sensed with the at least one electrode during the secondsensing period.
 2. A method as defined in claim 1 wherein the firstrecited establishing step includes isolating an R wave from the sensedventricular activity and commencing the first sensing period with thebeginning of the isolated R wave.
 3. A method as defined in claim 2wherein the first recited establishing step includes terminating thefirst sensing period after the isolated R wave.
 4. A method as definedin claim 2 wherein the second recited establishing step includesterminating the second sensing period before the beginning of the firstsensing period.
 5. A method as defined in claim 4 wherein said secondsensing period is terminated about 40 milliseconds before the beginningof the first sensing period.
 6. A method as defined in claim 4 whereinthe second recited establishing step includes beginning the secondsensing period about 150 to 200 milliseconds before the beginning of thesecond sensing period.
 7. A method as defined in claim 4 wherein thesecond recited establishing step includes beginning the second sensingperiod a predetermined time before the termination of the second sensingperiod.
 8. A method as defined in claim 1 further including determininga first peak amplitude of the heart activity sensed during the firstsensing period and a second peak amplitude of the heart activity sensedduring the second sensing period, and wherein the comparing stepincludes comparing the second peak amplitude to the first peakamplitude.
 9. A method as defined in claim 1 including an initial stepof determining that the heart is in normal sinus rhythm.
 10. A method asdefined in claim 9 including repeating the recited steps at spaced aparttimes.
 11. A method as defined in claim 9 including repeating therecited steps once each day.
 12. A system for detecting dislodgment ofan implantable right atrial endocardial lead having at least oneelectrode, the system comprising: means including an electrode pair forsensing ventricular activity of the heart within the right ventricle ofthe heart; means for establishing a first sensing period responsive tothe ventricular activity sensed with the electrode pair; means forestablishing a second sensing period spaced from and preceding the firstsensing period established responsive to the ventricular activity sensedwith the electrode pair; means for sensing activity of the heart withthe at least one electrode of the implantable endocardial lead duringthe first sensing period and the second sensing period; and means forcomparing the heart activity sensed with the at least one electrodeduring the first sensing period to the heart activity sensed with the atleast one electrode during the second sensing period and for detectingdislodgment of the right atrial endocardial lead by said comparison. 13.A system for detecting dislodgment of an implantable right atrialendocardial lead having at least one electrode, the system comprising:an electrode pair; a ventricular activity detector having an inputconnected to the electrode pair and an output; an amplifier having aninput connected to the at least one electrode and an output; amicroprocessor connected to the output of the ventricular activitydetector and to the output of the amplifier and including an R windowstage to establish a first sensing period responsive to a signalprovided on the output of the ventricular activity detector, a P windowstage to establish a second sensing period spaced from and preceding thefirst sensing period, and a comparison stage to compare a signalprovided on the output of the amplifier during the first sensing periodwith a signal provided on the output of the amplifier during the secondsensing period and to detect dislodgment of the right atrial endocardiallead by comparison.
 14. The system for detecting dislodgment of animplantable right atrial endocardial lead of claim 13 wherein themicroprocessor includes an R peak stage to determine a first peakamplitude of the signal provided on the output of the amplifier duringthe first sensing period and a P peak stage to determine a second peakamplitude of the signal provided on the output of the amplifier duringthe second sensing period, and wherein the comparison stage compares thefirst peak amplitude to the second peak amplitude.